The present invention relates to a process for and an apparatus for the catalytic cracking of hydrocarbon loads.
It is known that in oil refineries, cracking processes are normally used in which the molecules of hydrocarbons of high boiling point and molecular weight are split into smaller molecules with a lower boiling point.
In the case of recent catalytic cracking processes, described for example in the patent EP-A-291253, the cracking reaction takes place in an elongate enclosure of substantially circular cross-section, the catalyst being admitted into the bottom part of the enclosure, together with the previously atomized hydrocarbon load. The establishment of contact between the load and the heated catalyst makes it possible to vaporize the hydrocarbons which then entrain the catalyst to the upper part of the reaction zone, the introduction of a propellant fluid assisting the ascending movement.
It is therefore necessary to vaporize the load appropriately in such processes. Indeed, if one is to obtain products of a cracking process which have a boiling point below that of the load, it would be prejudicial if this atomized load were to come in contact again with catalyst which has already given off a major part of its heat. Not only would the efficiency of the reaction be affected thereby but the retromixture of the catalyst would likewise produce additional coking.
Partly in order to limit these drawbacks, catalytic cracking processes have been conceived in which a reaction medium is used which follows a descending flow path. The catalyst is then introduced into the upper part of the reaction zone and the atomized hydrocarbon load is injected into the same part but in the descending flow of the catalyst (patent documents U.S. Pat. No. 4,385,985 and EP-A-254 333).
The product of reaction is then subjected to separation, for example in a cyclone, in order to obtain on the one hand, a gaseous hydrocarbon phase and, on the other, the catalyst. The catalyst is regenerated and then at least partially recycled to the cracking reactor.
These descending flow processes are satisfactory with regard to the reaction but using them poses problems. One drawback resides in the solid/gas separation by virtue of the substantial concentration of catalyst in the reaction medium, which requires specifically designed equipment if suitable efficiency is to be enjoyed. Furthermore, industrial installations generally comprise an ascending flow type of reaction chamber. The considerable and troublesome modifications to be made in order to change the direction of flow dissuade the use of these efficient processes.